Prepregs for use in building lay-ups of composite materials

ABSTRACT

A scrim-reinforced prepreg assembly for use in building low-porosity lay-ups. The assembly includes a prepreg that is composed of a fibrous reinforcement and a heat-curable resin mixture, the prepreg having suitable viscosity and sufficient tackiness to hold a scrim which is adhered to the prepreg by applying light pressure. The scrim is impressed onto the prepreg to such a degree, that less than half of the circumference of the scrim strands becomes coated by the heat-curable resin mixture.

This application is a divisional of U.S. patent application Ser. No.12/724,457, which was filed on Mar. 16, 2010 and which is a divisionalof U.S. patent application Ser. No. 11/122,453 which was filed on May 5,2005 and issued as U.S. Pat. No. 7,709,404.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to composite materials and in particularto prepregs which can be used in building composite materials of lay-upsof such prepregs which are pressmolded and cured.

2. Description of Related Art

The use of composite materials, in particular reinforced thermosettingcomposites, is continually increasing; a new application of suchmaterial is for windmill blades in wind energy plants.

Prepregs have been known and used for this purpose, which are composedof heat-curable resins and fibers and may also comprise a coarse net ofreinforcing rovings. Such prepregs are sold to the customer who can formcomposite materials of different shapes by building up lay-ups of suchprepregs, for instance 50 and more prepreg layers, and press molding andheating these lay-ups to obtain the appropriate shape and cure theresin.

In highly stressed components the void content of such laminates issignificant for the performance and therefore for dimensioning suchparts, as each void is a point of defect which decreases the mechanicalproperties. For this reason the customer requires prepregs which producea low, reproducible void content, but which at the same time have goodhandling properties.

Since air tends to be captured between several layers of prepregs, ithas been customary to process the lay-up of the prepregs under vacuum.It has also been known to intermittently interpose dry, air-permeablelayers of for instance resin-free webs between the resin layers to allowthe air to escape through these dry layers when putting on vacuum. Thistechnique is rather troublesome and does not yield reproducible results,since in the heating step the resin penetrates the air-permeable layerirregularly. A technique of this kind is described in DE 202 01 902 U1,whereby the prepreg is combined with a web which over its thickness isonly partially impregnated. It is difficult to fix such a thick web tothe prepreg without applying a further resin layer at the top, possiblytogether with another fixing element, otherwise there will be loosefilaments at the outer side of the prepreg, which would impair thehandling properties.

SUMMARY OF THE INVENTION

The inventors now have found a simple but very efficient way to allowthe air to escape during the press-molding step using a scrim or veillike material as a means to provide air escape paths. Surprisingly, areduction in laminate void levels and hence improved mechanicalproperties were found. A thermoplastic scrim or veil is the preferredmaterial although alternatives such as glass or natural fiber scrim,fabric or fleeces are also suitable. Grid weights of 60 gsm or lower aredesirable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section view of an exemplary embodiment of the presentinvention.

FIG. 2 is a cross section view of another exemplary embodiment of thepresent invention.

FIG. 3 is a top view of an exemplary embodiment in which the strandsextend over the edge of the prepreg.

FIG. 4 is a graph of a typical viscosity profile of resins in accordancewith the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In its general form the invention therefore provides a process formaking a scrim-reinforced prepreg for use in building low-porositylay-ups, whereby a conventional prepreg is formed of a reinforcement anda heat-curable resin, the prepreg having suitable viscosity andsufficient tackiness to hold a scrim which is adhered to the prepreg byapplying only light pressure, so that the scrim is impressed onto theprepreg to such a degree, that less than half, preferably less than 30%,and more preferably less than 25% of the circumference of the scrimstrands become coated by the prepreg resin i.e. the scrim is primarilyon the surface of the prepreg web. A typical viscosity profile for sucha resin is shown in graph 1 (FIG. 4). Tack, which is a measure of theadhesion of a prepreg ply to the tool surfaces or to other prepreg pliesin the assembly, is an adhesion characteristic of the matrix resin thatis controlled in order to facilitate ply cutting and lay-up operations.The plies should be capable of being removed and repositioned ifnecessary. For the purpose of this invention, a suitable tack level isone that allows two prepreg plies to stick together when one is placed,by hand, on top of the other on a flat surface and, also allows the twoplies to be subsequently separated by light hand pressure.

This procedure leaves more than the upper half of the scrim strandsuncoated with the resin, so that when the next prepreg is put on top ofthe scrim of the lower prepreg, channels are formed along the strands ofthe scrim, through which the air can escape when the vacuum is appliedduring further processing and before these channels are closed by themolding pressure.

The light pressure to be applied in order to impress the scrim onto theprepreg to the required degree can be easily and efficiently obtained bysimply laminating the scrim to the prepreg during the wind-up step onthe prepreg machine. There is adequate tension in the wound-up roll tocomplete the partial impregnation into the prepreg. Such techniques arewell known to those skilled in the art.

Suitable heat-curable resins for the preparation of the prepreg areselected The heat-settable resin mixtures of the present inventioninclude a resin component and a curing agent component. The resincomponents includes one or more thermosetting resins. Exemplary resinsinclude epoxy, cyanate ester, polyester, vinyl ester and bismaleimideresins. Exemplary epoxy and cyanate ester resins include: glycidylaminetype epoxy resins, such as triglycidyl-p-aminophenol,tetraglyidyldiaminodiphenyl-methane; glycidyl ether type epoxy resins,such as bisphenol A type epoxy resins, bisphenol F type epoxy resins,bisphenol S type epoxy resins, phenol novolak type epoxy resins, cresolnovolak type epoxy resins and resorcinol type epoxy resins; and cyanateesters, such as 1,1′-bis(4-cyanatophenyl)ethane (e.g. AroCy L-10,available from Vantico, Inc., Brewster, N.Y.),1,3-Bis(4-cyanateophenyl-1-1-(1-methylethylidene)benzene (e.g. RTX366,available from Vantico, Inc., Brewster, N.Y.).

Epoxy resins are preferred. The epoxy may be composed of trifunctionalepoxy, difunctional epoxy and a wide variety of combinations oftrifunctional and difunctional epoxies. Tetrafunctional epoxies may alsobe used as can aliphatic and alicyclic epoxies. Exemplary trifunctionalepoxy include triglycidyl p-aminophenol andN,N-Diglylycidyl-4-glycidyloxyaniline (MY-0510 or MY-0500 available fromVantico, Inc., Brewster, N.Y.). Exemplary difunctional epoxies which maybe used in the resin include Bis-F epoxies, such as GY-281, LY-9703 andGY-285 which are available from Vantico, Inc., Brewster, N.Y.). Bis-Aepoxies, such as GY-6010 (Vantico, Inc., Brewster, N.Y.). Epon 828(Resolution Performance Products) and DER 331 (Dow Chemical, Midland,Mich.) are suitable Bisphenol-A type epoxies and may also be used. Anexemplary tetrafunctional epoxy is tetraglycidyl diaminodiphenyl methane(MY-721, MY-720 and MY-9512 available from Vantico, Inc., Brewster,N.Y.). Preferred bis-F epoxies include GY281 and GY285 which areavailable from Vantico. Inc., Brewster, N.Y. Other commerciallyavailable epoxies that have been used in making composite materials arealso suitable.

The curing agent component can include any of the known curing agentsfor thermoset curing of resins. The curing agents may be used alone orin combination as is well known. Suitable curing agents include:anhydrides; Lewis acids, such as BF3; amines such as dicyandiamide;3,3-diamino-diphenylsulfone (3,3-DDS); amino or glycidyl-silanes such as3-amino propyltriethoxysilane; CuAcAc/Nonylphenol (1/0.1);4,4′-diaminodiphenylsulfone (4,4′-DDS);4,4′-methylenebis(2-isopropyl-6-methylaniline), e.g., Lonzacure M-MIPA(Lonza Corporation, Fair Lawn, N.J.);4,4′-methylenebis(2,6-diisopropylaniline), e.g., Lonzacure (Lonza Corp.,Fair Lawn, N.J. Substituted ureas or imidazoles may also be useful ascuratives.

The curing temperature of the curable resin mixture will depend upon theparticular curing agents and resins being used and the relative amountsof each. In general, the resin(s) and curing agent(s) will be selectedso that the curing temperature will be less than 200° C. A preferredcure temperature range is between 75-120° C.

Further minor ingredients may be included as performance enhancing ormodifying agents in the matrix resin composition, such as any of thefollowing: accelerators; thermoplastics and core shell rubbers; flameretardants; wetting agents; pigments/dyes; UV absorbers; anti-fungalcompounds; fillers; toughening particles and viscosity modifiers.

The reinforcing fibers may be synthetic or natural fibers or any otherform of material or combination of materials that, combined with theresin composition of the invention, forms a composite product. Thereinforcement web can either be provided via spools of fiber that areunwound or from a roll of textile. Exemplary fibers include glass,carbon, graphite, boron, ceramic and aramid. Preferred fibers are carbonand glass fibers. Hybrid or mixed fiber systems may also be envisaged.The use of cracked (i.e. stretch-broken) or selectively discontinuousfibers may be advantageous to facilitate lay-up of the product accordingto the invention and improve its capability of being shaped. Although aunidirectional fiber alignment is preferable, other forms may also beused. Typical textile forms include simple textile fabrics, knitfabrics, twill fabrics and satin weaves. It is also possible to envisageusing non-woven or non-crimped fiber layers. The surface mass of fiberswithin the fibrous reinforcement is generally 80-4000 g/m², preferably100-2500 g/m², and especially preferably 150-2000 g/m². The number ofcarbon filaments per tow can vary from 3000 to 320,000, again preferablyfrom 6,000 to 160,000 and most preferably from 12,000 to 48,000. Forfiberglass reinforcements, fibers of 600-2400 tex are particularlyadapted.

The thickness of the fibrous filaments may range from 10-100 microns.

The resin content in the prepreg is of some importance and preferablyshould be between 25 and 45 weight %, most preferably 29 to 35 weight %.The resin content is also dependent on the fibrous material in theprepreg. Usually glass fibers require a lower resin content than carbonfibers. With glass fibers a preferred resin content is between 25 and 38weight % and with carbon fibers between 27 and 42 weight %.

Although the resin viscosity range can be rather broad, in general, theviscosity of the resin is between 5×10³ to 5×10⁵ Pas sec. at ambienttemperatures of 20-25° C.

The wide-meshed scrim or grid may be made of any suitable material, butthermoplastic yarns are preferred. The key requirement of the yarnmaterial is that it has a melting point similar to or higher than theprepreg gelling temperature so that the scrim yarns do not melt duringthe curing process. Preferably, the difference between yarn melt pointand the matrix gelling point should be at least 10° C. Suitablematerials for the scrim include polyester (76-1100 dtex) such aspolyethylene terephthalate and polybutylene terephalate and copolymersthereof, polyamide (110-700 dtex) such as nylon 6, nylon 66, nylon 10,nylon 11 and nylon 12, polyethersulphone, polypropylene, viscose stapleyarn (143-1000 dtex), meta and para-aramid (Kevlar® 29 220-1100 dtex andNomex® T-430 220-1300 dtex, glass 220-1360 dtex), jute (2000 dtex), flax(250-500 dtex), cotton (200-500 dtex) and combinations of one or more ofthese Such material is available under the Bafatex tradename fromBellingroth GmbH, Wipperfuerth, Germany.

The strands which form the scrim preferably have a substantially roundcross-section. The diameter of these strands preferably may be 100 to1000 μm, preferably 200 to 600 and more preferably 300 to 400 μm. If thescrim fiber diameters are too large, then mechanical properties of thecured laminate may be adversely affected. For example, bothinter-laminar shear strength and compression strength were found todecrease.

The essential feature of the invention is that the strands of the scrimare not fully impregnated by the resin of the prepreg. The degree bywhich the strands of the scrim are coated with resin can be expressed bythe degree of impregnation (DI). The DI indicates to which degree thecircumference of the scrim strands are covered with resin. Therefore, animpregnation index of 1.0 means that the strands are fully impregnatedby the resin and an impregnation index of 0.5 indicates, that half ofthe circumference of the grid strands is coated by the resin. Theinvention requires that the scrim strands are covered with the prepregresin to a minimum degree, just sufficient in order that the scrim willadhere to the prepreg to assure safe handling. It must not be covered bythe resin, however, to 50% of the circumference of the strands or more,to assure the proper provision of air escape channels. Therefore,expressed as a “degree of impregnation”, the invention requires that thedegree of impregnation is between >0 and <0.5 and preferably between 0.2and 0.3.

To assure that the outward ends of the air channels provided along thescrim strands do not become clogged by the prepreg resin, the scrimshould extend outwardly beyond the edges of the prepreg. Preferably thescrim should jut out over the edges of the prepreg by 2 to 30, inparticular by 10 to 20 mm.

A polyethylene or silicone coated release paper may be placed as aprotector layer on one or both sides of the prepreg—scrim assembly.

The manufacture of the prepregs is usually done by way of running sheetswound up on rolls although supply of cut sheets is also possible. Thewidth of the material can be between 10 and 2000 mm, preferably between200 and 1100 mm. Lengths of several hundreds of meters are conventional.

The structure of the scrim is of importance and consists of two mainelements. In the 0° or warp direction, the yarns are used to primarilystabilize those yarns that are aligned in other directions even whenunder tension in a wound-up roll. Other yarns, that run in a crosswisedirection to the warp yarns form parallelograms. In general the gridforms a coarse net in which the parallel strands of the parallelogramhave a distance of 3 to 60, preferably 10 to 35 and most preferably 20to 30 mm from each other.

For the escape of the air the short channels to the lateral edges of theprepreg formed by the strands in roughly cross-direction are ofimportance. Preferably the scrim should include parallelograms with sidelengths of 10 to 35 mm, wherein the smaller angle of the parallelogramis between 50 and 80, preferably between 65 and 75°. Therefore the scrimpreferably should comprise strands in longitudinal direction, which isthe running direction of the sheet, and strands in roughlycross-direction to the running direction of the sheet. With such a scrimconstruction during the pressmolding of the prepreg lay-ups, alsoadvancing in longitudinal direction, the air will first advance alongthe longitudinal strands up to a point, where the longitudinal strandmeets a strand in cross-direction, from where the air will escapeoutwardly along a strand in cross-direction. These strands incross-direction create a short way outwardly. In this connection, alsothe angle between the strands in longitudinal direction and the strandsin roughly cross-direction is of practical importance.

In a preferred embodiment the prepregs consist of 65 to 71 parts byweight of unidirectionally aligned carbon fibers fully impregnated with29 to 35 parts by weight of a thermosettable resin, to which prepregthere is impressed a scrim consisting of a) longitudinal i.e. warpstrands having a distance from each other of 3 to 12 mm and b) strandsin roughly cross-direction forming parallelograms with a smaller angleof 65 to 75 degrees and a side length of 10 to 35 mm, the strands havinga substantially round cross-section and a diameter of 200 to 600 μm,whereby the strands of the scrim are impressed into the prepreg to sucha degree, that 2 to 40% of their circumference are impregnated with theresin of the prepreg, and whereby 10 to 20 mm of the grid jut out overthe lateral edges of the prepreg.

The prepregs according to the invention are particularly useful for themanufacture of low-porosity lay-ups for windmill blades.

Comparative Experiments

The advantages of the invention are demonstrated by the following fourcomparative experiments, in which the result of experiment C accordingto the invention is compared with the result of experiments A, B andaccording to the prior art.

Experiment A

Alternate plies of carbon prepregs with a resin content of 40% and 24%by weight, each with 150 g/m² carbon UD fibers were put one on top ofthe other (50 layers of each kind of prepreg, in total therefore 100layers) to form a lay-up. The matrix resin type was M9.6 available fromHexcel, Pasching, Austria and the fiber type was T600S available fromSoficar, Abidos, France. This lay-up was subsequently pre-compacted andcured under vacuum, then cut and tested for the presence of air voids. Atwo-step cure cycle was used in which the temperature was slowlyincreased to 85° C. over 2 hours 15 minutes and then held at 85° C. fora further 1.5 hours. This was followed by a further temperature ramp to120° C. over 1.5 hours with a second temperature hold at 120° C. for 1hour. Cooling to 90° C. and preferably to below 60° C. is desirablebefore removing the cured laminate from the mold.

Experiment B

Experiment A was repeated with the only difference that 50 layers ofcarbon prepregs with a resin content of 33% by weight and a content of300 g/m² carbon UD fibers were put one on top of the other.

Experiment C

A lay-up was formed of 100 carbon prepreg layers exactly like inexperiment A with the only difference that a polyester scrim wasattached to each prepreg when the lay-up was formed from the prepregs.This polyester grid had a construction of 160 yarns per 100 cm in 0°direction with the diagonal yarns having an angle of 70° and aseparation of 25 mm. The scrim was impressed on the prepreg such that adegree of impregnation of 0.2 to 0.4 was obtained.

Experiment D

A prepreg was prepared consisting of the same polyester scrim as used inexperiment C, carbon UD fibers in an amount of 500 g/m² and 32% byweight of the same resin. Scrim and fibers were fully impregnated withthe resin. With this prepreg material a lay-up of 50 layers was formedand cured and tested as in experiments A to C.

Results

Cured laminates from the above experiments were sectioned through thethickness, the surfaces finely ground and visual observations made onthe void content. Experiments A and B resulted in a low amount of airvoids in the cured lay-up. Experiment C resulted in a laminate which waspractically free of air voids. In the laminate of the experiment Dnumerous and large air voids had been visible.

The invention is further demonstrated by the attached FIGS. 1 to 3.

FIG. 1 shows a cross-section of a prepreg (1) consisting ofunidirectionally aligned fibers (2) and resin (3). Impressed on thisprepreg is a scrim consisting of strands (4).

FIG. 2 shows a prepreg (1.1) identical with the prepreg shown in FIG. 1,on the top of which there is a second prepreg (1.2) of the same kind,whereby already some pressure had been applied, so that the air channels(5) at the sides of the scrim strands (4) become visible.

FIG. 3 shows the construction of a scrim consisting of longitudinalstrands (4 l) and strands in roughly cross-direction (4 c), thesestrands extending over the edges of the prepreg (1).

What is claimed is:
 1. A prepreg assembly comprising a prepregcomprising a fibrous reinforcement comprising reinforcing fibers and aprepreg resin comprising a thermoset resin that is curable at a curingtemperature, said prepreg having a surface that has tack and comprisessaid prepreg resin and wherein the surface mass of said reinforcingfibers is from 80 g/m² to 4000 g/m², and a scrim located on said surfaceof the prepreg, said scrim comprising a plurality of thermoplasticstrands that each have a cross-sectional diameter of 100 microns to 1000microns, said thermoplastic strands forming form a grid ofparallelograms comprising warp direction strands and strands that arecrosswise to said warp direction strands, said warp direction strandsbeing spaced from 3 to 60 millimeters from each other and wherein saidstrands that are crosswise to said warp direction strands are from 3 to60 millimeters from each other, said thermoplastic strands each having across-sectional diameter and a circumference which is determined by saidcross-sectional diameter, wherein less than half of the circumference ofeach of said thermoplastic strands is coated by said prepreg resin andmore than half of the circumference of each of said thermoplasticstrands is not coated with said prepreg resin and wherein said prepreghas sufficient tackiness to hold said scrim in place on the surface ofsaid prepreg.
 2. A prepreg assembly according to claim 1 wherein saidthermoplastic strands each have a cross-sectional diameter of 200microns to 600 microns.
 3. A prepreg assembly according to claim 2wherein said thermoplastic strands each have a cross-sectional diameterof 300 microns to 400 microns.
 4. A prepreg assembly according to claim1 wherein less than 30% of the circumference of each of saidthermoplastic strands is coated by said prepreg resin.
 5. A prepregassembly according to claim 1 wherein said fibrous reinforcement in saidprepreg comprises fibers that are unidirectionally aligned.
 6. A prepregassembly according to claim 1 wherein the thermoplastic strands have asubstantially round cross-section.
 7. A prepreg assembly according toclaim 1 wherein said prepreg comprises an edge and wherein said scrimextends outwardly beyond said edge.
 8. A prepreg assembly according toclaim 1 wherein the curing temperature of said prepreg resin is between75° C. and 120° C.
 9. A prepreg according assembly according to claim 1wherein said prepreg resin has a gelling temperature and wherein themelting point of said thermoplastic strands is at least 10° C. higherthan said gelling temperature.
 10. A prepreg assembly according to claim1 wherein said thermoset resin is selected from the group consisting ofepoxy resin, cyanate ester resin, polyester resin, vinyl ester resin andbismaleimide resin.
 11. A prepreg assembly according to claim 10 whereinsaid thermoset resin is epoxy resin.
 12. A prepreg assembly according toclaim 1 wherein said thermoplastic strands are selected from the groupconsisting of polyester, polyamide, polyethersulfone and polypropylene.13. A prepreg assembly according to claim 12 wherein said thermoplasticstrands are polyester.
 14. A prepreg assembly according to claim 11wherein said thermoplastic strands are polyester.
 15. A cured compositepart that comprises a prepreg assembly according to claim 1 which hasbeen cured.
 16. A prepreg assembly according to 1 wherein saidreinforcing fibers have a surface mass of from 150 g/m² to 2000 g/m² andconsists of carbon tows comprising from 12,000 to 48,000 filaments. 17.A prepreg assembly according to claim 16 were said carbon tows areunidirectionally aligned.
 18. A prepreg assembly according to claim 1wherein said fibrous reinforcement consists of glass fibers which have alinear mass of from 600 tex to 2400 tex.